top view 

the above is a tank design for saving fry the gravid female is put into the compartment on the left with some plants and a small sponge filter when the fry are born the flow of the water push's them into the right compartment away from the mother lowering the chances of being eaten

design by Ivan Dibble i believe

the above design is meant to sit on top of a larger tank with the v part in water the larger livebearer is placed it in and the fry fall out of the bottom i have seen this design with the runners as above and with the runners along the sides of the v 

i first saw  this design being used by Jim Montgomery 

i would advise you not to use the livebearing traps from shops as they are too small and the fry can be sucked out of the grids at the ends by other fish if in an emergency  use a 9inch net inside a 10 inch net with some floating plants floating in a tank